1. Technical Field
Embodiments of the present disclosure relate generally to amplifiers, and more specifically to a low-noise amplifier with tuned input and output impedances.
2. Related Art
Low-noise amplifiers (LNA) are commonly used to amplify an input signal and provide a corresponding amplified output signal with minimal addition of noise. LNAs are often employed in environments in which matched impedances are required both at the input and output of the LNA. For example, LNAs are typically used at the front-end of communication receivers, and may need to handle input and output signals of high frequencies. Wired paths (for example, from an antenna) connected to an input terminal of an LNA typically behave as transmission lines and are associated with a characteristic impedance at the frequency-band(s) of interest. Similarly, wired paths connecting the output terminal of an LNA to another component in the signal chain may also behave as a transmission in the frequency band(s) of interest. Thus, both the input impedance of the LNA as well as the output impedance of the LNA may need to be matched to (i.e., tuned to be equal to) the characteristic impedance of the transmission line(s). Further, an LNA may be required to provide the (same) matched input and output impedances over a range of frequencies, typically representing a frequency-band of interest.
Input impedance of a circuit (such as an LNA), as is well-known in the relevant arts, is the ratio of a voltage applied at the input terminal of the circuit and the resulting current drawn from the source providing the voltage. The output impedance is generally the impedance in series with a voltage source representing the output portion of the circuit. Output impedance may be represented as the Thevenin equivalent impedance looking into the output terminals of a circuit.
Some prior LNAs use resistors as part of the impedance matching network that provides the matched impedance (input impedance or output impedance). As is well known in the relevant arts, resistors add to noise due to thermal effects, and such an approach may not be desirable. Some other LNAs use a relatively large number of components to provide the tuned input and output impedances, and may not be desirable at least for this reason.